A variety of devices require high peak power pulses during operation. Typical devices using pulse power technology include particle beam accelerators, high-power microwave devices, high energy lasers, nuclear effects simulators and fusion devices. The technology field is replete with switching devices capable of high power, and typical of these is the device known as the spark gap switch.
A simple spark gap switch consists of two electrodes separated by an insulating gas. Often, one electrode is hollow having a trigger pin located inside the electrode. This trigger pin is used to initiate the main spark discharge using a low-energy pulse. Such devices can handle millions of volts and hundreds of kiloamps in low repetition rate applications (less than one hertz). In the usual operation of the switch, a spark forms, heating the surrounding gas and causing the switch to "close" and conduct electricity. The voltage at which the switch closes is the breakdown voltage. This breakdown voltage is dependent on pressure and temperature of the gas in the spark gap. If a second power pulse is applied to the switch before cooling can take place, the switch will close at a much lower voltage. The requirement for consistent operation of the switch at a specific breakdown voltage limits the repetition rate of the switch. The repetition rate is limited by the period of time required to rid the gas of the excess heat. This period is called the recovery time. The recovery time limitation means that low-energy spark gaps have been able to operate at high repetition rates, but high-power switches have been typically limited to about 10 Hz.
A variety of techniques have been used to allow higher repetition rates. One approach has been to use blowers to move hot gases out of the switch region. Above 1,000 Hz, the necessary cooling requires supersonic gas flow. The large blowers needed to provide such a flow result in a switch system that is very large and inefficient.
Spark gap switches are used in a wide variety of high-power applications requiring high currents and voltages. Low repetition rate has been the major limitation in using spark gap switches in rep-rated, high-power systems. This low repetition-rate, or lack-of-recovery, occurs because upon switch closure, a hot conductive channel forms in the interelectrode gas. This channel heats the gas and causes a reduction in the gas-particle number density. The gas cools by a variety of processes and given enough time will reach the initial particle density and hence the initial voltage holdoff strength. This recovery time is relatively short if hydrogen is used as a fill gas, but improvement is necessary for high-repetition systems.
In prior art devices, electrodes act primarily as passive electrical contacts to the gaseous switch medium. The electrodes may provide some cooling for the conductive channels formed in the gas switch but do not actively help the recovery of the inter-electrode gas. Normally, electrodes are made of some high-refractory material (stainless steel and copper-tungsten alloys for instance) to improve electrode erosion and lifetime.
Other prior art spark gap switches exhibit special triggering to improve timing or use saturated vapor or liquid between the electrodes in an effort to reduce jitter and inductance.